# Pore system shapes

Series Exploring for Oil and Gas Traps Treatise in Petroleum Geology Predicting the occurrence of oil and gas traps Predicting reservoir system quality and performance Dan J. Hartmann, Edward A. Beaumont Web page AAPG Store

## Archie and non-Archie rocks

Choquette and Pray's[1] porosity types include two different groups of pore system shapes: petrophysically simple Archie porosity and petrophysically complex non-Archie porosity. In most cases, water saturation (Sw) of rocks with Archie porosity can be predicted from log analysis using the Archie equation

${\mbox{S}}_{{{\rm {w}}}}=\left({\frac {1}{\Phi ^{{{\rm {m}}}}}}\times {\frac {{\mbox{R}}_{{{\rm {w}}}}}{{\mbox{R}}_{{{\rm {t}}}}}}\right)^{{{\rm {1/n}}}}$

where:

• Sw = water saturation of the uninvaded zone
• n = saturation exponent, which varies from 1.8 to 4.0 but normally is 2.0
• Rw = formation water resistivity at formation temperature
• Φ = porosity
• m = cementation exponent, which varies from 1.7 to 3.0 but normally is 2.0
• Rt = true resistivity of the formation, corrected for invasion, borehole, thin bed, and other effects

without modification. To predict water saturation in rocks with non-Archie porosity, we modify the Archie equation.

## Table of characteristics

The table below describes pore system shapes and other important characteristics of Archie and non-Archie rocks.[2]

Feature Archie Non-Archie
Pore system shapes Intergranular (found between rounded particles); interparticle Mold-like
Intercrystalline (found between angular particles) Vug-like
Fracture-like
Relationship of pore shape to rock particles Negative image of particles making up matrix Relates only indirectly to particles making up matrix
Pore connectivity Pore throats connect pores into regular networks Pores are irregularly distributed and can be either poorly or very well connected
Porosity reduction processes Grain coating or pore filling by calcite, silica, or dolomite Pore or pore throat filling by clays or other minerals